Semiconductor laser modules in which light emitted from a plurality of semiconductor lasers is coupled into an optical fiber have been conventionally available. Inside a housing of the semiconductor laser module, a plurality of semiconductor lasers are provided side by side. This means that, as the number of installed semiconductor lasers increases, the length of the semiconductor laser module increases. Thus, more compact semiconductor laser modules have been in demand.
As such a semiconductor laser module, there is a semiconductor laser module, for example, in which a plurality of lasers are arranged so as to face each other, with lenses and mirrors corresponding to each of the semiconductor lasers being arranged in a plurality of tiers in a height direction, and coupled into an optical fiber (U.S. Pat. No. 8,432,945, for example).
FIG. 8 is a plan view showing a conventional semiconductor laser module 100 disclosed in Patent Document 1. FIG. 8 is an upper perspective view of a housing 103. The semiconductor laser module 100 mainly includes the housing 103, semiconductor lasers 105, lenses 107 and 109, reflecting mirrors 111, a condenser lens 115, an optical fiber 119, and so on.
The semiconductor lasers 105, the lenses 107 and 109, the reflecting mirrors 111, the condenser lens 115, the optical fiber 119, and the like are disposed inside the housing 103. The semiconductor laser module 100 has a plurality of columns of the semiconductor lasers 105 disposed to improve installation efficiency of the semiconductor lasers 105. That is, a plurality of semiconductor laser installed columns 121a and 121b are provided.
A base portion of the housing 103 is in a staircase-like form so as to increase its height. Each step serves as a semiconductor laser installation platform 117. The semiconductor laser 105 is installed on each of the semiconductor laser installation platforms 117.
Also, the semiconductor lasers 105 in each of the semiconductor installed columns 121a and 121b are disposed on the semiconductor laser installation platforms 117 at different heights. Thus, the semiconductor lasers 105 disposed in the semiconductor installed columns 121a and 121b respectively are disposed at all different heights with a uniform pitch.
In each of the semiconductor installed columns 121a and 121b, the lenses 107 are disposed in front (in an emitting direction) of the semiconductor lasers 105. Also, the lenses 109 are disposed further ahead thereof. The lenses 107 and 109 collimate light emitted from the semiconductor lasers 105 in vertical and horizontal directions, respectively. The lenses 107 and 109 are disposed for the every semiconductor laser 105.
Here, the semiconductor lasers 105 in each of the semiconductor installed columns 121a and 121b are disposed so as to face each other and radiate laser beams toward the center of the housing 103 in a width direction thereof. At the center of the housing 103, the reflecting mirrors 111 are disposed approximately in a line. The reflecting mirror 111 is disposed on the installation platform at the same height as the each corresponding semiconductor laser 105.
The light emitted from the semiconductor lasers 105 in the semiconductor installed columns 121a and 121b and collimated by the lenses 107 and 109 is reflected on the reflecting mirrors 111 and changes its direction approximately perpendicularly. At this time, the light emitted from the semiconductor lasers 105 in the semiconductor installed columns 121a and the light emitted from the semiconductor lasers 105 in the semiconductor installed columns 121b are reflected in a same axis direction when viewed from above.
The light reflected by the respective reflecting mirrors 111 (a beam group 123) is condensed by the condenser lens 115 and optically coupled to the optical fiber 119.
FIG. 9 is a schematic view showing an arrangement of the semiconductor laser 105 and the reflecting mirror 111. Light radiated by the semiconductor laser 105 (an arrow X in the drawing) has a predetermined angle of beam divergence (8 in the drawing). This divergence is collimated by the lens 109 and then reflected on the reflecting mirror 111 (an arrow Y in the drawing).
Here, the installation efficiency of the semiconductor lenses 105 is highest when an installation pitch for the semiconductor lasers 105 (pD in FIG. 8) is equal to a width of the semiconductor laser 105 (WS in FIG. 9). Also, the semiconductor lasers 105 in each of the semiconductor installed columns 121a and 121b are disposed being shifted for half a pitch and facing each other.
Thus, to avoid interference between the reflecting mirrors 111 that reflect the light from right and left respectively, it is required that an effective reflecting width SA of the reflecting mirror 111 is less than half of WS (=pD). For example, when a thickness of the reflecting mirror 111 is 0, SA=pD/2. However, when the thickness of the reflecting mirror is considered, the effective reflecting width SA of the reflecting mirror 111 becomes less than pD/2.
As above, it is impossible to have an enough wide effective reflecting width SA of the reflecting mirror 111 in the conventional semiconductor laser module. This leads to a problem that the semiconductor laser 105 with large θ cannot be used.
On the other hand, if the reflecting mirror 111 with a wider width is to be used, it is required to increase the installation pitch pD of the semicondutor laser 105 larger than the width WS of the semiconductor laser 105. That is, it is necessary to provide a clearance between the semiconductor lasers 105.
However, this decreases the installation efficiency of the semiconductor lasers 105. The semiconductor lasers 105 are divided into the semiconductor laser installed columns 121a and 121b and light from the semiconductor lasers 105 is condensed to improve the installation efficiency of the semiconductor lasers 105. However, if the semiconductor lasers 105 are disposed being kept apart, then it is impossible to obtain sufficient effects from providing a plurality of the semiconductor installed columns 121a and 121b. 
On the other hand, to cope with the narrower effective reflecting width SA of the reflecting mirror 111, there is a way to decrease a distance between the semiconductor laser 105 and the lens 109 (EFLSAC in FIG. 9).
FIG. 10 is a schematic view showing a relationship between the semiconductor laser 105 and the lens 109. As mentioned above, light X radiated from the semiconductor laser 105 has the predetermined angle of divergence a θ. This light X enters into the lens 109 within a range that is determined by θ and EFLSAC.
Here, the light enterering a proximity of an edge portion of the lens 109 in a width direction thereof is not collimated completely and may cause a deviation. This deviation is called spherical aberration. Spherical aberration is known to be proportional to 1/R2 of a spherical surface of the lens 109. Thus, to reduce the spherical aberration, it is required to increase R.
However, to increase R, it is effective to increase EFLSAC. Thus, to reduce the spherical aberration, it is required to have sufficient EFLSAC. This means that, in order to reduce the influence of the spherical aberration, it is difficult to decrease the distance EFLSAC between the semiconductor laser 105 and the lens 109.
Also, as shown in FIG. 8, in the conventional method, a step pitch of the semiconductor laser installation platforms 117 in the respective semiconductor laser installed columns 121a and 121b is different from a step pitch of the installation platforms for the reflecting mirrors 111. Thus, adjusting the each reflecting mirror 111 is difficult and assembly is difficult.
For these reasons, easy-to-assemble semiconductor laser modules with improved installation efficiency of the semiconductor lasers 105 and small optical deviation due to spherical aberration are demanded.